Dirty air
Dirty air is the turbulent wake behind a car, which strips downforce from a following car and makes it harder to corner closely.
A Formula 1 car generates grip from airflow over its bodywork. Following closely disrupts that flow, so the chasing car has less grip precisely where it needs most — through the corner before a straight.
Reducing the effect was the stated aim of the 2022 ground-effect regulations. It also explains why the slipstream helps on the straight while dirty air hurts in the corner: the same wake, doing opposite things.